如何在Flutter中的Future函数之外获取变量值？

Question

我正在开发一个下载pdfs的书籍应用程序，我遇到问题，无法通过pdfviewer获取下载书籍的完整路径。

我有一个函数将路径传递给一个名为path的变量但是当我打印它时它是null，如果我热重新加载我得到了正确的路径。

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
import 'package:flutter_pdf_viewer/flutter_pdf_viewer.dart';

class PdfScreen extends StatefulWidget {

  final DocumentSnapshot document;

  PdfScreen(this.document);

  @override
  _PdfScreenState createState() => _PdfScreenState(document);
}

class _PdfScreenState extends State<PdfScreen> {

  final DocumentSnapshot document;
  dynamic path;
  var dir;

  _PdfScreenState(this.document);


  Future<void> getPdf() async{

    try{

        dir = await getApplicationDocumentsDirectory();

        path = "${dir.path}/${document["title"]}.pdf";



    } catch(e){
      print(e);
    }


  }

  @override
  Widget build(BuildContext context) {

    getPdf();
    print(path); // I need the path here


 // the Path I need:    //"/data/data/br.com.apps.d.baixa_livros/app_flutter/${document["title"]}.pdf";

    return Container();
  }

}

Answer 1

这应该工作。当你获得path时，你需要setState以便重建小部件。

class _PdfScreenState extends State<PdfScreen> {

  final DocumentSnapshot document;
  dynamic path;
  var dir;

  _PdfScreenState(this.document);

  @override
  void initState() {
    super.initState();
    getPdf();
  }

  Future<void> getPdf() async{
    try{
        dir = await getApplicationDocumentsDirectory();
        setState(() {
          path = "${dir.path}/${document["title"]}.pdf";
        });
    } catch(e){
      print(e);
    }
  }

  @override
  Widget build(BuildContext context) {

    if (path == null) {
      print("loading"); // show a progress indicator
      return CircularProgressIndicator();
    } else {
      print(path); // I need the path here
      return Container();
    }

  }

}